This invention relates to vehicle steering systems, more particularly to such systems where variations in mechanical load are sensed during a steering manoeuvre to provide a signal which serves for monitoring or control purposes. The invention was primarily developed for use with power assisted steering gears for motor vehicles.
It has hitherto been proposed to provide devices in a steering mechanism which are responsive to variations in mechanical loads in the system caused by effecting a steering manoeuvre and to provide an output from such devices which can be used to control power assistance. Several such devices have been proposed; a first of these uses a potentiometer to measure the mechanical loads whereby a slider on the potentiometer is connected by a linkage to a member in the steering mechanism (the load on which member it is desired to detect) so that the slider is moved by the linkage in proportion to the load--thereby changing the electrical resistance of the potentiometer and ultimately to provide a required output signal. In the case of power assisted steering mechanisms the output signal which results from the change in electrical resistance can be used to control power assistance which may be provided by servo motors to the system. This type of known device usually employs a slip ring potentiometer which is mounted on the input shaft of the steering gear. Because of safety requirements this type of device requires very careful and trouble free construction and is consequently expensive, furthermore it has the disadvantage that the moving contacts and linkage of the potentiometer are susceptible to wear and interference for example by corrosion and extraneous matter.
A second type of known device depends upon the carbon pile effect for its operation whereby the load to be measured is transmitted to an appropriate material (such as carbon) to stress that material and thereby change its electrical resistance, the changes in resistance can be detected in a similar fashion to the first type of device and hence the load can be determined.
A third type of known device (as exemplified by British Specification No. 2,030,094A) employs a "force rod sensor" which enables either one of two electrical circuits to be closed (depending on the direction of load applied) and this serves to indicate to a control device the direction of the load so that the control device can respond accordingly.
In the prior proposals as discussed above, a fault in the system (such as a failure of an electrical connection) may be interpreted as a change in the load which is being sensed, and for a steering system it will be appreciated that this is most undesirable.
It is an object of the present invention to provide a steering system which alleviates the disadvantages of the prior proposals and lends itself to a reliable, compact and inexpensive structure for detecting and responding to variations in mechanical load effected by a steering manoeuvre whereby an output signal is provided which can be used for monitoring or control purposes associated with the steering.